1. Field of the Invention
The present invention relates to an AC/DC converter provided with a power-factor improvement (PFC) unit and a current resonance (LLC) converter unit, more particularly to a switching power unit that can achieve highly effective performances through a power-factor improvement operation and an output voltage control by changing the on-duty and the switching frequency of a full-bridge type switching element.
2. Description of the Related Art
For digital apparatuses or home-use amusement equipments such as laptop computers, liquid crystal televisions, plasma televisions, game machines, a switching power unit that is provided with an AC/DC converter in which to improve a power-factor has been used. In general, the switching power unit is composed of a full-wave rectifier bridge, a booster type power-factor improvement (PFC) unit and a DC/DC converter portion.
For the DC/DC converter portion, a flyback type converter, a forward type converter, a current resonance (LLC) type converter, etc. may be named. Here, when a highly effective power source is required, the current resonance type converter has been widely applied.
In FIG. 8, a half-bridge type switching power unit is disclosed. See Japanese Patent Application Laid-Open No. 2008-283818 (Patent Document 1). This half-bridge type switching power unit is composed of a full-wave rectifier bridge 18, a power-factor improvement portion 20, and a half-bridge type current resonance converter 30.
In the circuit structure of the half-bridge type switching power unit, the power-factor improvement portion 20 has an active filter including an inductor 21, a diode 22 and a switch element 23 as well as a smoothing capacitor 26, between the full-wave rectifier bridge 18 and two switching elements 31, 32 provided on the input side of the current resonance converter 30.
Further, in the current resonance converter 30, a series resonant circuit (a resonant capacitor 33 and a resonant inductor 34) is connected between the intermediate point of series circuit of the switching elements 31, 32 and the primary winding of a transformer T. Current that flows toward the secondary winding of the transformer T is rectified and smoothed by rectifier diodes 35, 36 and a capacitor 37 so as to obtain predetermined output voltages.
In this circuit, the switching elements 31, 32 alternatively conduct On-Off operations through a frequency controller 38. Switched high frequency voltages are applied to both ends of the primary winding of the transformer T and then output to a secondary side through the transformer T thus being converted into DC output voltages.
This switching power unit can provide high conversion efficiencies to the current resonance converter 30; however, the switching power unit is structured by three multi-stage circuits composed of: the full-wave rectifier bridge 18; the power-factor improvement portion 20; and the current resonance converter 30. Accordingly, the overall efficiency of the device is generally decreased to approximately 85 to 90%.
Considering the above, the present inventor, as shown in FIG. 9, invented a half-bridge type switching power unit that has improved conversion efficiencies in such a manner as to eliminate a full-wave rectifier bridge and to commonly utilize the switching elements of both a power-factor improvement unit and a current resonance converter.
This switching power unit includes an AC/DC converter circuit composed of a power-factor improvement (PFC) portion 20a and a current resonance converter (LLC) portion 30a. In the power-factor improvement portion 20a, the series circuit of first and second diodes D1, D2 is connected in parallel with the series circuit of first and second switching elements Q1, Q2 in a forward direction. At the intermediate point between the both series circuits, a booster inductor L1 and an AC source Vac are connected in series. Further, a first smoothing capacitor (an electrolytic capacitor) Ci is connected on the both sides of the series circuit of the first and second diodes D1, D2 in parallel to the first and second switching elements Q1, Q2.
The current resonance converter 30a includes: first and second switching elements Q1, Q2 of a half-bridge circuit sharing the series circuit of the first and second switching elements Q1, Q2 of the power-factor improvement portion 20a; a series resonant circuit 6 including a resonant inductor Lr and a resonant capacitor Cr placed on the primary side between the half-bridge circuit and a high frequency transformer T; and a rectifier circuit including rectifier diodes D3, D4 and a second smoothing capacitor Co on the secondary side of the transformer. The series resonant circuit 6 is connected in series with the primary winding of the high frequency transformer T.
The power-factor improvement portion 20a performs as a booster circuit as that when AC power-supply voltages are in a positive half cycle (hereinafter, when the intermediate point side of the first and second switching elements Q1, Q2 is a positive voltage, this condition will be referred to as the “positive half cycle”), energy stored in the booster inductor L1 when the second switching element Q2 is ON is transferred to the smoothing capacitor Ci when the second switching element Q2 is OFF. Further, when the AC power-supply voltages Vac are in a negative half cycle, energy stored in the booster inductor L1 when the first switching element Q1 is ON is transferred to the first smoothing capacitor Ci when the first switching element Q1 is OFF.
In this switching power circuit, since there is no full-wave rectifier bridge, further high efficiency can be obtained compared to the switching power circuit disclosed in the Patent Document 1. See FIG. 8. In addition, since a number of parts can be minimized, it would be possible to structure a circuit with less cost.
However, since the above-described half-bridge circuit is designed as that the on-duty of the switching elements is approximately 50%, it would be difficult to control the power-factor improvement unit that shares the switching elements. For example, even when loads are relatively small, the on-duty of the switching element cannot be further narrowed. Accordingly, the PFC voltages of the power-factor improvement unit excessively increase, whereby there is a risk that the switching elements or the capacitor may be damaged.
Further, in conventional power-factor improvement circuits, there are a critical current mode or a continuous current mode. In these modes, the power-factor improvement operations are adapted to perform by changing the on-duty of switching elements according to the phase angles of AC input voltages. However, considering a switching power circuit disclosed in FIG. 9, since the on-duty of switching elements is fixed to approximately 50%, when the instantaneous value of the AC input voltages and the value of PFC voltage come closer, it would become difficult to appropriately perform power-factor improvement operations.
On the other hand, for example, as a full-bridge type switching power unit that improves power-factors while reducing a number of parts, Japanese Patent Application Laid-Open No. 2006-79985 (Patent Document 2) discloses the structure of electronic ballast for use of electric discharge lamps. See FIG. 10.
In FIG. 10, a power unit 40 is composed of a power-factor improvement unit (a converter circuit unit) 41 and a full-bridge circuit unit (an inverter circuit unit) 42. With 4 switching elements Q1 to Q4, an electric power is suppliable to electric discharge lamps while improving its power-factor. This power unit 40 performs the boost of input voltages by changing the on-duty of first and second switching elements Q1 and Q2. Based on the on-duty, timing to supply electric current to the electric discharge lamps is controlled, so that it would be possible to supply high output powers to the electric discharge lamps while improving its power-factor.
The circuit structure of this power unit 40 is composed of: the converter circuit unit 41 to rectify and boost input voltages from an AC source Vac; the inverter circuit unit 42 that supplies rectangular-wave output voltages to an electric discharge lamp 45; a detecting means 46 that detects voltage/current information of the converter circuit unit 41 and the inverter circuit unit 42; and a control means 47 that performs the switching control of the switching elements Q1 to Q4 included in the converter circuit unit 41 and the inverter circuit unit 42. Further, although 4 switching elements Q1 to Q4 are used, the converter circuit unit 41 and the inverter circuit unit 42 partially share these switching elements.
Considering power-factor improvement operations in this case, the first and the second switching elements Q1 and Q2 alternatively repeat ON-and-OFF operations. When AC input voltages are the positive half cycle, energy is to be stored in a choke coil while the second switching element Q2 is ON. When the second switching element Q2 is OFF, energy that has been stored in a choke coil L1 is transferred to a first smoothing capacitor Ci.
Here, for power-factor improvement, (when the continuous current mode is applied), the on-duty of the second switching element Q2 is adapted to change according to the phase angle of the AC input voltages. An ON width will be widened when the input voltages are low. On the other hand, when the input voltages are high, the ON width will be narrowed. That is, when the phase angle is nearly 0 degree or 180 degrees, the on-duty of the second switching element Q2 is widened. To the contrary, when the phase angle is 90 degrees, it is controlled to make the on-duty to become the minimum. Here, as regards the first switching element Q1, when the phase angle is nearly 0 or 180 degrees, the on-duty is narrowed, whereas when the phase angle is 90 degrees, the on-duty is widened. In case that the AC input voltages are the negative half cycle, the role between the first and the second switching elements Q1 and Q2 will be reversed.
In the inverter operation of the switching elements Q1 to Q4, when the first and the fourth switching elements Q1 and Q4 are ON, negative electrode voltages are output to lamp loads. When the second and the third switching elements Q2 and Q3 are ON, positive electrode voltages will be output. Since the switching elements Q1 and Q2 also work for power-factor improvement operations, on-duty is changed according to phase angles. When the phase angle is nearly 90 degrees on the positive half cycle, the on-duty of the second switching element Q2 is small (on the contrary, the on-duty of the first switching element Q1 is large).
Accordingly, if positive inverter operations are performed approximately at the phase angle of 90 degrees on the positive half cycle, sufficient power is not suppliable to the load due to small on-duty of the second and the third switching elements Q2 and Q3. Based on the above, approximately at the phase angle of 90 degrees, the negative inverter operations should be performed in combination of the first and the fourth switching elements Q1 and Q4 that have large on-duty, to supply necessary power to the load.
When the phase angle is nearly 0 degree or 180 degrees, since the on-duty of the switching element Q2 becomes larger, positive inverter operations will be performed in combination of the second and the third switching elements Q2 and Q3. Here, when AC input voltages are the negative cycle, operations will be reversed relative to the above.
Accordingly, in this circuit, by changing polarities of the inverter outputs according to the on-duty (that is, the phase angle of the AC input voltages) of the first and second switching elements Q1 and Q2, sufficient output powers will be suppliable.
In this circuit, however, since the polarities of the inverter outputs are determined according to the phase angle, when the phase angle is approximately 90 degrees, only negative polarities are allowed for output. Further, when the phase angle is approximately 0 degree or 180 degrees, only positive polarities are allowed for output. Here, when the AC input voltages are the negative half cycle, operations become reversed. Note that the output voltages of inverter are generally the positive polarity and the negative polarity in asymmetry. The general inverter outputs positive polarities and negative polarities in an alternative manner; however, the above operations are not in conformity therewith. The operations do not thus necessarily fit to optional loads.
As described hereinabove, since the switching power unit shown in FIG. 9 is the half-bridge type, the on-duty becomes fixed to approximately 50%. Accordingly, it would be difficult to control the power-factor improvement unit that shares the switching elements. On the other hand, the switching power unit of Patent Document 2 shown in FIG. 10 shows the full-bridge type. However, the polarities of the inverter outputs are determined based on the phase angles of the AC input voltages. This type of the unit does not apply the method of outputting positive and negative polarities in an alternative manner like general inverter operations. Here, voltage waveforms will be asymmetric pulse waveforms.